The ABRs and ASBR Propagation of LSAs

When a router is configured as an ABR, it generates summary LSAs and floods them into the backbone area. Routes generated within an area are Type 1 or Type 2, and these are injected as Type 3 summaries into the backbone. These summaries are then injected by the other ABRs into their own areas, unless they are configured as totally stubby areas. Any Type 3 or Type 4 LSA received from the backbone will be forwarded into the area by the ABR.

The backbone will also forward external routes both ways unless the ABR is a stub router, in which case they are blocked.

If a summary is received from within the area, it cannot be forwarded, and summaries received from the backbone cannot be further summarized.


The different types of areas mentioned in this section are described in the later section "The Different Types of Areas."

Figure 6-4 The Propagation of LSAs

Figure 6-4 The Propagation of LSAs

It is necessary for certain conditions to be met before any LSAs can be flooded out of all interfaces:

• The LSA was not received through the interface.

• The interface is in a state of exchange or full adjacency.

• The interface is not connected to a stub area (no LSA Type 5 will be flooded).

• The interface is not connected to a totally stubby area (no Type 3,4, or 5 will be propagated).

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